Particle measuring instruments for measuring particles entrained in liquids, need to maintain a relatively constant rate of flow through the particle sensor of the instrument to achieve accurate and consistent measurement of particle sizes. One way for maintaining a constant flow is to pump the liquid through the particle sensor by means of a positive displacement pump such as a gear pump which if run at a constant rate can achieve a relatively constant rate of flow. However a gear pump is not effective to maintain a sufficiently constant rate of flow when the liquid entraining the particles is at a high pressure or is at relatively high viscosities because these conditions will cause a pressure variation across the gear pump which in turn will cause a variation in the leakage through the gear pump and thus cause a variation in the rate of flow through the sensing instrument.